How can I track how many miles the user has walked?

Question

I have made some java code in android studio to where it counts how many steps someone takes but I'm not sure how I can make it to where it counts how many miles the user walks. Here is the code.

If anyone can tell me how to do it that would be appreciated, Thanks.

    package com.example.andyheggis.healthapp;

import android.content.Context;
import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity implements SensorEventListener {


    TextView tv_steps;

    SensorManager sensorManager;

    boolean running = false;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);



        tv_steps = (TextView) findViewById(R.id.tv_steps);

        sensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);
    }

    @Override
    protected void onResume() {
        super.onResume();
        running = true;
        Sensor countSensor = sensorManager.getDefaultSensor(Sensor.TYPE_STEP_COUNTER);
        if(countSensor != null) {
            sensorManager.registerListener(this, countSensor, SensorManager.SENSOR_DELAY_UI);
        }  else {
            Toast.makeText(this, "Sensor not found!", Toast.LENGTH_LONG).show();
        }
    }

    @Override
    protected void onPause() {
        super.onPause();
        running = false;
        //if you unregister the hardware will stop detecting steps
        //sensorManager.unregisterListener(this);
    }

    @Override
    public void onSensorChanged(SensorEvent event) {
        if(running) {
            tv_steps.setText(String.valueOf(event.values[0]));
        }
    }

    @Override
    public void onAccuracyChanged(Sensor sensor, int accuracy) {

    }
}